Housings for explosion protected lamps which meet the safety requirements of the International Electrotechnical Commission (IEC) "enhanced safety" standard IP 54, must also meet the following requirements: They must be protected against dust in such a way that ingress of dust, although it may not be totally prevented, does not enter in sufficient quantity to interfere with satisfactory operation of the equipment, here the lamp; and it must be protected against water splashed against the enclosure from any direction, without harmful effect- on the electrical component within the lamp. To permit such lamps to be secured to a support surface, for example the ceiling of a room to be illuminated, it has been customary to form the lamp housing as a single injection molded plastic structure of essentially trough shape, covered by a transparent cover to permit light to be projected from the structure. The trough-like structure retains the electrical components, such as ballasts, switches, sockets for the lamps, radio suppression equipment, and the like, as well as associated wiring. These trough-shaped structures are quite long, and, especially, the longitudinal dimension thereof is large with respect to the wall thickness of the plastic housing structure.
Manufacture of such housing structures is difficult since, based on manufacturing problems, numerous restrictions on the shape of the structure must be accepted, and compromises must be made.
The undesirable and unfavorable relationships between wall thickness and overall dimension inhibits the use of reinforcement fibers, such as glass fibers; or, at best, permits the use of only small glass fibers. Eliminating or substantially reducing fiber reinforcement also substantially reduces the overall strength; the strength of the housing is substantially less than housings which are reinforced with long glass fibers. An injected molded housing of this type must be filled with injection molding material from a plurality of inlets so that the overall composition of the structure, when it is finished, will be non-homogeneous. This, further, reduces the overall strength. If any air bubbles are entrained, during injection, which is very difficult to detect, further weaknesses in the housing will result.
Undercuts, T-grooves and the like, are preferably avoided at the upper part of the trough-shaped housing structure. Such undercuts can be made only by using slider tools in the molds. Such molds are very expensive and are substantially more subject to malfunction or incorrect molding than molds which are free of movable parts. To prevent undercuts, it is necessary to so form the upper parts of the housings to have many edges and projections since, otherwise, undercuts would arise, or that spaces are filled with plastic material which, further, increases production costs.
Based on production problems, the transparent cover can be hinged to the upper trough-like housing portion of the light fixture only at two or three spaced points. The resulting hinged portions, formed on the housing structure and on the transparent cover, are so constructed that the cover is hooked into the housing structure or housing portion. In use, these hinges can become unhooked, which requires some dexterity on part of operating personnel when it is intended to exchange light bulbs, typically fluorescent tubes. These hinge portions, further, are of comparatively low strength and, when the cover is open, are subject to breakage. The closing force with which the cover is closed against the trough-like housing portion, required if the overall structure is to meet the safety requirements of "increased safety" as defined by the international standards (as well as by coordinate ANSI standards)is just barely adequate.
From a production point of view, it is particularly annoying that for each size of lamp fixture, for use with specific sizes of light sources, for example 30 W, 40 W or 60 W fluorescent tubes, it is necessary to provide individual molds, which are very expensive.
In spite of the above disadvantages, explosionprotected housings were usually made of single-element housing portions,although it has previously been proposed to make housings for light sources by fitting together extrusion parts and end caps, for placement in ordinary, potentially non-hazardous environment. Thus, international standard IP 10, for example,protects the lamp only against surfaces greater than 50 mm, that is, large surfaces such as a hand, casually engaging against the light, without any special protection with respect to water.